A fuel cell stack has a stack structure with a plurality of cells stacked in series. Each of the cells includes a membrane-electrode assembly with an anode located on one surface of an electrolytic membrane and a cathode located on the other surface thereof. Supplying a fuel gas and an oxidation gas to the membrane-electrode assembly induces an elecrtrochemical reaction. Thus, chemical energy is converted into electric energy. In particular, a solid polyelectrolyte fuel stack using a solid polymer film as an electrolyte requires reduced costs, is easy to make compact, and offers a high power density. The solid polyelectrolyte fuel stack is thus expected to be used as a vehicle-mounted power source.
To operate a fuel cell system, moisture inside the fuel cell stack and the temperature of the interior thereof need to be closely managed to detect a power generation error such as a shortage of a supply of a reaction gas caused by flooding or an increase in resistance caused by the dried electrolytic membrane. Thus, the power generation condition of the cells needs to be managed so as to provide sufficient power. Japanese Patent Publication No. 7-63020 refers to a scheme of controlling operation so as to prevent an electrode potential from increasing above an upper voltage limit while preventing a cell voltage from decreasing below a lower voltage limit (the electrode potential refers to the potential of each of the anode and the cathode, and the cell voltage refers to the potential of the cathode with reference to the anode). When the fuel cell stack continues to be operated at a high potential at which the electrode potential is in an oxidation region, a catalyst may be ionized and eluted. Thus, the upper voltage limit is specified so as to prevent the electrode potential from increasing enough to degrade the catalyst. Furthermore, when power generation is continued with some of the cells offering a reverse potential (in this phenomenon, the anode potential is higher than the cathode potential), the catalyst degradation or the like may occur to deteriorate power generation performance. Thus, the lower voltage limit is specified so as to prevent the possible reverse potential.    [Patent Document 1] Japanese Patent Publication No. 7-63020